Pent roof shed felting

fifthofwhisky

I have a pent roof shed with T&G panels on the roof. The panels are treated with a wood preserver/stain (leftovers). Size approx 6 x 8 feet. I purchased some quality roof felt (IKO Green Ultra Durable Super Shed Felt).Can I lay the felt directly on the roof or do I need (or is it advisable) to use any kind of breathable membrane or underlay?

Niv

Personally I think that EDPM is the way to go, felt is rubbish by comparison. However, if you insist on using felt, yes it is suitable to lay it directly on the roof, I am not sure what point a breathable membrane would have in this scenario tbh.

Section62

fifthofwhisky said:

I have a pent roof shed with T&G panels on the roof. The panels are treated with a wood preserver/stain (leftovers).

The problem with T&G (although marketed as a superior product) is the individual planks can move relative to each other (e.g. due to changes in moisture content) which can result in the felt puckering or being torn.

I'd prefer to lay any sheet roofing onto a manufactured board (e.g. waterproof OSB or ply) which is dimensionally more stable.  The other advantage of this is if leaks do develop in the coating the water is more likely to run off the bottom of the board (outside) rather than through T&G joints inside. Subject to material compatibility, a layer of polythene between the board and sheet roofing can give a further degree of waterproofing.

The other thing is if any of the wood preserver/stain is oil-based then you need to make sure it is completely dry (full VOC evaporation) before putting felt directly on it.

fifthofwhisky

Thanks for the replies. I'm recycling an old shed hence using the T&G leftovers (otherwise I'd go for the sheet). Tried to keep the cost down but ended up spending a lot of time! 

The preserver is dry from VOC for sure. Need to dry out a bit with recent rains.

I do have a polythene sheet so might use that over the T&G and under the felt. 

chrisw

I wouldn't bother with the polythene. It will trap water and condensation and might encourage rot whereas the felt will allow the wood to breathe.

Section62

fifthofwhisky said:

I do have a polythene sheet so might use that over the T&G and under the felt. 

chrisw said:

I wouldn't bother with the polythene. It will trap water and condensation and might encourage rot whereas the felt will allow the wood to breathe.

I guess that depends whether the OP is going to try sourcing breathable shed felt.


Note that the product already purchased by the OP does state that for any roof with a low pitch (between 10° and 20°) "a waterproofing underlay is required".  So if the roof is less than 20 degrees it will need something more than just the felt applied directly to the T&G.

fifthofwhisky

Not sure of the exact angle but it's probably around 15 degrees. So I need a 'waterproofing underlay'. Will a breathable membrane sheet do, or do I need an underlay felt?

grumbler

IMO, There is no point in putting breathable membrane between timber boards and felt (or similar).

If you want to make it more reliable, there are special underlays for felt, e.g.

And I'd avoid using nails and use felt adhesive instead.